This invention concerns new and advantageous adhesive compositions which are particularly useful for bonding wood, metal, and plastics.
Phenolic-formaldehyde resins and modifications thereof have long been used as strong, water-resistant adhesives for making plywood and other bonded wood products. The curable intermediate stage resins useful in adhesive compositions comprise two general types, those basically of novolac or bisphenol kind of structure but which have reactive methylol groups on the aromatic rings, prepared by reacting a phenol with formaldehyde in the presence of an acidic condensation catalyst, and resoles, especially those having a preponderance of methylol groups in ortho relationship to a phenolic hydroxyl group, which are made by reacting a phenol with formaldehyde in the presence of a neutral or basic catalyst. Both types are curable to insoluble, infusible resins, usually by application of heat and, particularly in the case of those having a methylolated novolac structure, by reaction with additional formaldehyde.
Aziridine resins of various types are known to be good adhesives, but these also generally require heat curing at an elevated temperature.